It has long been a desire to reduce drag caused by the passage of bodies through fluid media. Relatively small reductions in drag can significantly reduce the fuel needed to propel a body. For example, it has been estimated that a one percent reduction in drag across the leading edge of a wing of a Boeing 727 airliner could reduce fuel consumption by more than 20,000 gallons per airplane per year. See Automotive Engineering, Feb., 1982, pp. 73.
The reduction of drag across surfaces of a body also is of benefit in other applications. By way of example, it is desirable to reduce the drag caused by water flowing past the hull of a boat, air flowing past a moving automobile or air flowing past the blades of a windmill fan, airfoil, fan, rotor, stator, inlet, etc. Many other examples are of course known as will be appreciated by those in the art. However, as yet there has not been provided a practical solution to the problem of reducing drag.
Many techniques have been proposed which involve mechanically altering flow control surfaces. For example, the utilization of various devices to direct air into ducts that exit at the trailing edges of the flow control surface has been suggested. See, for example, U.S. Pat. Nos. 2,742,247; 2,925,231; 3,117,751; 3,521,837; 4,114,836; 4,258,889; and 4,296,899.
U.S. Pat. No. 4,434,957 suggests the use of a corrugated control surface. The corrugations that extend transversely to the direction of the fluid flow, temporarily retain vortices formed in the fluid flow on the flow control surface, and aid in regulating their passage across the surface.
U.S. Pat. No. 4,455,045 proposes the use of one or more 3-sided submerged channels in the flow control surface. Each channel includes two divergent walls which form a generally V-shaped ramp which is sloped downward so that the channel widens and deepens toward the downstream flow of the fluid. Such channels are V-shaped in a plane generally parallel to the flow control surface. They are intricate and are most effective when provided in a serial cascade wherein the last channel in the cascade ends at the trailing edge of the flow control surface.
These techniques are expensive, time consuming to employ and do not address the problem of how to reduce drag across the surfaces of existing equipment (e.g., airplanes, automobiles, etc.) in a practical way.
The use of smooth surface coatings on airplane skins has also been suggested. See Automotive Engineering, Feb., 1982, pp. 73-78. However, this article reported that liquid polymeric coatings and adhesively backed films, applied to the flow control surfaces in order to maintain a smooth, protected surface for drag reduction, performed poorly and were unsuitable for areas of high erosion such as wing and tail leading edges and nacelle inlets.